You should definately check out Barcelona when you're in Spain. I loved Gaudi's architecture and its a very colourful city and the Sagrada Familia is just amazing, even if its not finished yet!
Other then that I must agree with what most people said here: Rome, Venice, Athens, London, Paris,Prague,...Free drugs in Amsterdam (though the city is very nice too)
Go to Montmartre by night, its cosy!
And since I'm the representative of Belgian Pride here... come visit Belgium, its a small country so you can get anywhere within 1 - 3 hours time (for 4euro's per trainride when you're under 26). Brussels is the Capital of Europe and a great place to start off, since you can get to Amsterdam, London and Paris within 2-3 hours by train.You'll love the food, best chocolate and beer in the world and you'll really like towns like Bruges, Ghent, Antwerp, Leuven, Durbuy, Bastogne, Waterloo, or my very own Mechelen. During the summer we have loads of music festivals and other activities and I wouldn't mind showing you around (I think I missed my calling as tourguide lol), and if I couldn't do it most (young) people in Flanders can speak English so you won't have any trouble getting around, we're very friendly
😉
Not convinced? check out a video our (currently non-existent) federal government made:
http://www.youtube.com/watch?v=snV_rDqkFxM
And If you're gonna dipinto africa, I can really recommend Djerba (Tunesia) for relaxing all in holidays... oh and Turkey is nice too, though not in Africa. Bodrum is a party-ville... oh oh and Egypt!!! *gets all excited*
anyone need a tourguide for Japan?